This application claims the priority of Japanese Patent Application No. 2002-193574 filed Jul. 2, 2002, in the Japan Intellectual Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a high-voltage power source apparatus that is installed in a developing unit of a printer using electrophotography in order to obtain an output voltage that is an overlap of a direct current (DC) voltage with an alternating current (AC) voltage.
2. Description of the Related Art
In general, a printer, which prints an image using electrophotography, illuminates a laser beam on a photosensitive drum to form an electrostatic latent image thereon, applies toner onto the electrostatic latent image to develop the image, and transfers the image coated with the toner onto transfer paper. A developing unit of such a printer includes a case in which toner is stored, and a developing roller in the case. To develop an electrostatic latent image, the developing unit makes the developing roller contact a photosensitive drum via a 0.2 mm aperture thereof, and then rotates the developing roller and the photosensitive drum in order to adhere the toner covering the developing roller to the electrostatic latent image on the photosensitive drum via the aperture.
For instance, a circumference of the photosensitive drum is charged with an electric potential of −50V and the other circumference of the photosensitive drum is charged with an electric potential of −700 V. Next, a voltage is generated by overlapping a DC voltage of −300 V with an AC voltage of 2000 Volts peak to peak (Vp-p) and the overlapped voltage is applied to the developing roller. As a result, the toner cleaves only to the surface of the photosensitive drum which is charged with the electric potential of −50 V. Accordingly, a general printer requires a high-voltage power source apparatus as shown in FIG. 1 which makes a voltage by overlapping the DC voltage with the AC voltage and supplies the obtained voltage to the developing roller.
FIG.1 is a circuit diagram of a conventional high-voltage power source apparatus that includes an AC voltage generator 100 and a DC voltage generator 200.
The AC voltage generator 100 comprises an operational amplifier OP1, a push pull output circuit having transistors Tr1 and Tr2 which are reciprocally connected to each other via bias resistors R1 and R2 and current limiting resistors R3 and R4, resistors R5 and R6 which form a voltage divider to bias a negative input of the operational amplifier OP1, a feedback resistor R7 connected between an output of the push pull output circuit and the negative input of the operational amplifier OP1, an input bias resistor R8, decoupling capacitors C1 and C2, a DC blocking capacitor C3 and a transformer T1.
The operational amplifier OP1 compares a pulse signal ACPWM (or a sine wave voltage) input to an input terminal 1 with a voltage which is obtained by adding a feedback voltage output from the push pull output circuit to a voltage determined by the resistors R5 and R6. Next, the operational amplifier OP1 outputs the result of the comparison to the push pull output circuit. The output of the operational amplifier OP1 is amplified by the push pull output circuit and output as an AC voltage to the DC blocking capacitor C3. The output AC voltage is stepped up by the transformer T1 and output as an AC voltage of 2000 Vp-p, which is similar to an input waveform, at a secondary side of the transformer T1. The capacitors C1 and C2 remove noise from input power sources, indicated as +24V and +5V, respectively.
The DC voltage generator 200 comprises a DC-to-DC converter which includes a controller 201 and a blocking oscillator 202. When a control signal CP is input to an input terminal 2 of the controller 201, a transistor Tr32 is switched on or off to cause the blocking oscillator 202 to oscillate or stop oscillating. An operational amplifier OP2 compares a reference voltage DCVref input through an input terminal 3 with a feedback voltage DCVfb input through an input terminal 4, and outputs the result of the comparison to a transistor Tr34. Then, the transistor Tr34 is controlled based on the comparison result to cause the blocking oscillator 202 to oscillate a frequency having a circuit constant value. A resistor R36 and a capacitor C34 filter the reference voltage DCVref which is input to the negative input of the operational amplifier OP2. A capacitor C35 decouples the positive input of the operational amplifier and diodes D32 and D33 limit the amplitude of the voltage DCVfb by clamping the voltage DCVfb to a power supply voltage +5V and to ground. A capacitor C36 and a resistor R37 provide feedback between an output and the input of the operational amplifier OP2. A resistor R38 couples the output of the operational amplifier OP2 to a base of the transistor Tr34. A collector of the transistor Tr34 is interfaced with the blocking oscillator 202 via a resistor R35 and a transistor Tr33 so that a collector voltage of the transistor Tr33 adjusts an internal reference voltage of the blocking oscillator 33 relative to a value established across a zener diode ZD31. The zener diode ZD31 is biased by current flowing from a power supply +24V through a resistor R31. A transistor Tr31 has a base connected to a common connection of the resistor R31 and the zener diode ZD31. An emitter of the transistor Tr31 is serially connected to ground via a resistor R33. A collector of the transistor Tr31 is protected from extreme negative voltages by a diode D31 which clamps to ground. A capacitor C31 provides decoupling of the +24V power supply. The 24V power supply is connected to one end of a primary coil of a transformer T1 and another end of the primary coil of the transformer T2 is connected to the collector of the transistor Tr31. A capacitor C32 is connected in parallel with the primary coil of the transformer T2. An auxiliary coil of the transformer T2 has one end connected to ground and another end which feeds back an induced current to the base of the transistor Tr31 through a capacitor C33 and a resistor R32. Respective polarities of the primary coil and the auxiliary coil are arranged so that the feedback through the transistor Tr31 results in an oscillatory voltage at the primary coil of the transformer T2, which couples an oscillatory output voltage to a coil on a secondary side of the transformer T2.
The oscillatory output voltage is extracted from the secondary side of the transformer T2, rectified and smoothed by a diode D34 and a capacitor C40 to provide a DC voltage. The DC voltage is is applied to a capacitor C5 connected in parallel with the secondary side of the transformer T1 through a resistor R43. The voltage applied to the capacitor C5 becomes a DC voltage of −300V, is overlapped with an AC voltage output from the transformer T2, flows through a protective resistor R10, and is output as an output voltage Dev through an output terminal 5. The output voltage Dev is applied to the developing roller.
When an output of the high-voltage power source apparatus of FIG. 1 is blocked, the resistor R11 discharges an electric current from the capacitor C5. Also, the DC voltage across the capacitor C40 is sampled by a voltage divider formed of resistors R41 and R42 to provide the voltage DCVfb, which is rectified and smoothed, is fed back to the input terminal 4 of the controller 201 as mentioned above. In addition, the pulse signal ACPWM, the control signal CP, and the reference voltage DCVref are respectively input to the input terminals 1, 2, and 3 at predetermined intervals, using a controller (not shown) included in a printer body.
As described above, a conventional high-voltage power source apparatus adopted by a developing unit of a general printer requires two high-voltage power source circuits, i.e., the AC voltage generator 100 and the DC voltage generator 200, thereby complicating the structure of the apparatus. For instance, the DC voltage generator 200 is a DC-to-DC converter including the controller 201 and the blocking oscillator 202 and therefore requires a large number of circuit elements as shown in FIG. 1. In particular, a color printer needs four DC voltage generators 200 for toners of four colors, i.e., yellow Y, magenta M, cyan C, and black B, thereby complicating the construction thereof and increasing the size and manufacturing costs.